There are some cases where it is required to connect two printed circuit boards (PCBs), two modules or two terminals (hereinafter, simply referred to as substrates as a general term) through high-speed digital signals while the two substrates rotate relative to each other. Examples include surveillance cameras, videos systems for monitoring the surroundings of a car, and robot joints.
In these cases, such a problem arises that wires are twisted as the two substrates rotate relative to each other when the two substrates are connected through the wires. Therefore, signal connection is achieved by pressing an electrode (ring) on one substrate against an electrode (brush) on the other substrate according to the prior art. In such a case, however, contact failure may be caused when the electrodes are worn away, the surface of the electrodes become rusty, or dust attaches to the surface of the electrodes. Therefore, maintenance and inspection are required, which increases costs.
When the characteristic impedance becomes disconnected at the contact point of the electrodes, part of a signal is reflected, which causes distortion to the signal that has passed. This becomes a cause of interference between symbols, which causes a problem where the speed of communication is prevented from increasing. In addition, adjacent brushes make contact with each other at the time of rotation in the case where distance between the terminals of a connector is narrowed, which prevents the miniaturization of the apparatus.
In order to solve these problems concerning the connector, the present inventor has proposed wireless communication of digital data by utilizing capacitive coupling and inductive coupling (together, referred to as electromagnetic field coupling) between transmission lines formed on the substrates that are placed in proximity to each other (see Patent Literature 1 through 3).
Patent Literature 1 proposes wireless communication between two substrates where differential transmission lines, consisting of two transmission lines that are arranged parallel to each other and are matching terminated, are arranged parallel to each other in the same direction. In this case, the shorter the length of the transmission lines is, the broader the band of the coupler is and the higher the speed of the data transfer can be. Alternatively, a pair of single-end (one differential) transmission lines can be used to provide wireless communication in the same manner.
In addition, Patent Literature 2 proposes wireless communication between two substrates where a pair of transmission lines are arranged in close proximity to each other when a differential signal is provided from the two ends of one transmission line. It is also indicated that the two substrates can be rotated relative to each other when the transmission lines are in arc form and the center axes are aligned. Furthermore, an example where one transmission line in arc form is shorter than the other has been proposed.
In addition, Patent Literature 3 proposes wireless communication between two substrates by directly coupling a pair of first transmission lines which are arranged parallel to each other and are matching terminated with a pair of second transmission lines which are arranged so as to face a pair of the first transmission lines.